Gg 1ggg

Voltage (Volts)

Figure 71. Radiation intensity and electric pumping current as functions of the pulse voltage. Reprinted with permission from [189], A. Blom et al., Appl. Phys. Lett. 79, 713 (2001). © 2001, American Institute of Physics.

was the concentration of 8 doping in the buffer and cap layers: 3 x 1011 cm"2 in their sample and 4 x 1011 cm—2 in the other ones.

At a low dc bias of 2 V the current I flowing parallel to the QW was measured as a function of the temperature. The result is plotted in the lower panel of Figure 70. Similar to the temperature behavior of the free hole concentration pv(T), the curve ln(I) vs 1/T also displays different characteristic features below and above T ^ 20 K. In fact, in the high temperature region, the slope of the ln(I) vs 1/T curve is almost the same as the slope of the ln(pv) vs 1/T curve. They then concluded that, in this temperature region, the dependence of the current I on 1/T was caused by the temperature dependence of the thermal population in the free hole levels but does not suggest an activation process as stated in [197, 199].

Then, they turned to the transport process in the low temperature region. Their calculated free hole concentration pv(T) shown in Figure 70, although small, is non-negligible at low temperatures. However, for such low temperatures and low concentration, the mobility may be very small [204], resulting in a decrease in the current of an order of magnitude. Nevertheless, they cannot rule out completely the possibility of hopping transport for the following reason. The charged impurity ions in both the buffer layer and the cap layer create a long range random potential in the QW. At the same time the spatial variation of the alloy composition produces a short range random potential. Both effects cause fluctuation of the 2D subband edge, and their calculations gave an energy fluctuation of about 5 meV. The transport of holes in the 2D subband would then exhibit hopping behavior with an activation energy of about 2-3 meV. It is important to emphasize that such a hopping process is entirely different from that stated in [197, 199], where the transport is ascribed to the hopping of holes from one acceptor to the other within the 8 layer in the well. Such a process requires partially charged acceptors. In conclusion in [189] Blom et al. calculated the number concentration of charged acceptors in the QW and plotted it as the dotted curve in the upper panel of Figure 70. At low temperatures, the number concentration of charge acceptors in the QW is far too small to produce a measurable hopping conductivity.

It can be emphasized that the mechanism of population inversion through the formation of resonant states is essential for the realization of semiconductor QW THz lasers. A full understanding of this mechanism will allow the design of advanced THz lasers using various lattice mismatched heterostructures with different semiconductors.

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